Polarizing optical elements that provide acceptable imaging qualities while maintaining durability and abrasion resistance are sought for a variety of applications, such as displays, windows, windshields, sunglasses, fashion lenses, non-prescription and prescription lenses, sport masks, face shields and goggles.
Conventional polarizing filters are formed from sheets or layers of a polymeric material such as polyvinyl alcohol that has been stretched or otherwise oriented and impregnated with an iodine chromophore or dichroic dye. Typically these impregnated sheets are layered between supporting films of cellulose triacetate or polyethylene terephthalate, which have good optical properties.
Polyurethane-containing materials such as polyurethane-ureas have been developed as useful polymers in the manufacture of optical articles because of their excellent properties such as low birefringence, resilience, and chemical and impact resistance. They have been used in mold castings for lenses, glazings, and the like. However, their use has been limited to these applications because of difficulties in preparing films of polyurethane-containing polymers. Such difficulties can include low gel time and high viscosity, thereby making conventional film casting of these materials very difficult due to poor workability.
Birefringence, or “double refraction”, in cast films can be caused by the orientation of polymers during manufacturing operations. The molecular orientation of the polymers may lead to significantly different indices of refraction within the plane of the film. In-plane birefringence is the difference between these indices of refraction in perpendicular directions within the plane of the film. Optical materials with low or negligible birefringence can be desirable in certain optical articles, in particular, in combination with polarizing filters to provide good optical properties.
It would be desirable to provide a method of preparing polyurethane-containing materials in free films, for use as film layers in optical elements and articles, so as to take advantage of their superior optical and mechanical properties.